1. Field of the Invention
This invention relates generally to the field of antennas, and more particularly to loop antenna assemblies intended for use in the high frequency and very high frequency portions of the electromagnetic spectrum.
2. Discussion of the Prior Art
The full wave loop antenna is generally recognized as one of the best conventional antennae for use in the high frequency and very high frequency portions of the electromagnetic spectrum for several reasons. First, the loop configuration tends to reduce fading on both transmission and reception. In addition, the full wave loop antenna provides enhanced directivity, gain, and xe2x80x9ccapture areaxe2x80x9d in comparison to other conventional antennae. Lastly, the full wave loop antenna is a xe2x80x9cquietxe2x80x9d receiving antenna and displays a reduced level of xe2x80x9cstaticxe2x80x9d interference when compared with other conventional antennae.
The two major types of full-wave loop antennae in wide use today in the high frequency and very high frequency spectrums are the xe2x80x9ccubical quadxe2x80x9d and the xe2x80x9cdelta quadxe2x80x9d. The cubical quad antenna is readily identified by the shape of its elements, which are generally square and vertically oriented. Similarly, the delta quad antenna may be identified by its elements, which are vertically oriented and generally in the shape of an equalateral triangle with one apex pointing toward the ground. The cubical quad normally uses support arms for the elements which join the mast at a single point, or a single boom which is fastened to the support arms, while the delta quad typically has all of the weight of the elements carried by a single boom.
Both the cubical quad and the delta quad antenna, however, display a number of disadvantageous characteristics.
First, due to the structural configuration of the antennae, they generally must be partially or completely assembled on the ground, making them quite difficult to install. For example, a typical cubical quad for the twenty meter amateur radio band has two square elements, each of which is approximately 17 feet on a side, which must be installed in a vertical orientation on a support structure such as a mast or a tower. Obviously, it is very difficult to assemble such a structure on the ground and then mount it on a support structure.
Second, the geometries of the cubical quad and delta quad make the antennae mechanically unstable, leaving them quite susceptible to dammage or destruction due to adverse weather conditions, such as high winds and snow and ice loading, when made from materials commensurate with their sizes, and light enough in weight to avoid the necessity of an oversized support structure.
Third, the cubical and delta quad antennae typically include a parasitic element (reflector), which is tuned to a resonant frequency which is two and one-half to three percent below the resonant frequency of the driven element with which it is designed to operate. Consequently, as the operating frequency of the antenna drops too far below the resonant frequency of the driven element, the parasitic element will approach self-resonance, substantially deteriorating the performance of the antenna. Also, the lengths of the sides of both the cubical quad and the delta quad elements are of equal or nearly equal length, thereby limiting the domain of reflector lengths at which co-incidence, or near co-incidence, of maximum foreward gain, maximum front-to-back ratio, and minimum VSWR (voltage standing-wave ratio) occur, both the lengths of the sides of the reflector elements and their total lengths being critical for efficient operation, the spacings between the driven element and the reflector element being adjusted to maximize the foreward gain on these antennae. The cubical quad and the delta quad are, therefore, limited to a relatively narrow range of operation frequencies below the resonant frequency of their driven elements, when using a reflector element to enhance their directional characteristics.
The present invention provides an improved full-wave loop antenna assembly having enhanced structural stability and effective directional characteristics over a wider range of frequencies than other antennas of its type. The antenna assembly is rotatable, self-supporting, may be made from commercially available materials, and is capable of supporting multiple driven and parasitic elements, in a variety of configurations, for multi-band operation. In addition, the antenna has semi-rigid element members which may be made from materials which are lighter in weight than those that would be used in the construction of xe2x80x9cYagixe2x80x9d-type antennae designed for use on the same frequency or frequencies, and can be installed by a single individual in much the same fashion as a xe2x80x9cYagixe2x80x9d type antenna.
In brief summary, the antenna assembly includes two essentially horizontal booms which are attached to an essentially vertical support structure. Conductive members, each of which forms a portion of either a driven or parasitic loop antenna element are attached to, and insulated from, the lower boom. A length of wire is attached to each member, near each end. A draw line, made of flexible insulating material, is fastened to each wire at, or nearly at, its center, fed through a bore in the upper boom, and through an eyelet mounted near the junction of the upper boom and the support structure for the booms, and run down next to the support structure to the lower boom, where it may be secured. Holding the draw lines in tension creates the desired geometric shape of the antenna elements.
Due to the configuration of the antenna assembly, the upper boom must always carry at least part of the weight of the antenna elements, as there is always some contact between the draw lines fastened to the wire portions of the largest antenna element installed on the structure and the upper boom, or some component which is fastened to the upper boom. For this reason, the upper boom will be referred to as the xe2x80x9cload-bearing upper boom.xe2x80x9d If sufficient tension is supplied to the aforementioned draw lines, all of the weight of the antenna elements may be removed from the lower boom. Releasing part of the tension on the draw lines allows part of the weight of the antenna elements to be carried by the lower boom, while still maintaining the desired geometric shape of the loop elements. Thus, the lower boom may, or may not be load-bearing for any particular loop element or group of loop elements, depending on the tension in the draw lines, the geometric configuration of the loop or loops, and the type of materials used for the loop element or elements. For this reason, the lower boom will be referred to as simply the xe2x80x9clower boomxe2x80x9d, although, with the exception of the test run on the embodiment depicted in FIG. 6, the lower boom was load-bearing in all embodiments disclosed.
The combination of each wire with its associated conductive member represents a loop-type driven or parasitic antenna element. Additional driven or parasitic elements may be added, and located either concentrically with other elements, or at different positions along the booms. In addition, a parasitic (reflector) element having a physical length substantially greater than those used by conventional loop parasitic reflector elements may be used to increase the effective operating bandwidth of the antenna, the optimum reflector length being a function of the separation (spacing) between the driven element and its associated parasitic reflector, a property not present in other full-wave loop antennas of this type. On all embodiments tested, the loop antenna elements were approximately one full wavelength in length at the frequencies being tested. No lumped circuit components, such as inductors or capacitors, or any other devices, were employed to electrically alter the lengths of the antenna elements. For brevity, the antenna elements will be referred to in this disclosure as simply the xe2x80x9cdriven elementxe2x80x9d and the xe2x80x9cparasitic elementxe2x80x9d, it being understood that all the antenna elements described in this disclosure are loop elements which are approximately one full electrical and physical wavelength in length at the frequencies being tested.
Tests run on all parasitic embodiments disclosed indicated an estimated foreward gain of approximately 5-8 db or higher, a front-to-back ratio of approximately 10-25 db or higher, and a front-to-side ratio of approximately 20-45 db or higher. The VSWR bandwidths of 2/1 or lower measured approximately 350 khz. or more on all embodiments disclosed, the antennas using a separate transmission line connected to the driven element indicating a considerably wider VSWR bandwidth of 2/1 or lower in most embodiments. The above and all references to xe2x80x9cdbxe2x80x9d refers to decibles in relation to a theoretical isotropic source. The antennas disclosed where mounted above a rooftop, with the lower boom at an elevation of approximately thirty-five feet.